Calcite

Calcite
Property	Value	Comments
Formula	CaCO₃
Crystal System	hexagonal
Crystal Habit	drusy, pisolitic, stalatitic, stalagmitic, prismatic, acicular, tabular; scalenohedrons & rhombohedrons most common; may be contact twinned; may also be cryptocrystalline or coarsely crystalline
Cleavage	perfect rhombohedral	rhombohedral cleavage is a distinguishing characteristic in thin section.
Fracture & Tenacity	conchoidal, brittle
Color/Pleochroism	colorless, white, pink, green, yellow
Optic Sign	uniaxial (-)
Optic Orientation	difficult to determine due to high birefringence	because calcite is uniaxial c must equal Z
Refractive Indices epsilon = omega =	1.486 1.658
Max Birefringence	0.172
Extinction	symmetrical to cleavage traces	when crystal is in an extinct position, birefringent calcite dust formed by grinding is visible.
Distinguishing Features	rhombehedral cleavage; high order color under xpl; very high relief; effervesces in dilute hydrochloric acid (hand sample); dolomite is different in that it contains cloudy inclusions with iron oxides.
Occurrence	Occurs in almost all rock types. It is the primary mineral in both marble and limestone. It occurs with dolomite, quartz, gypsum, barite, fluorite, pyrite, sphalerite, galena, chalcopyrite, danburite, stilbite, malachite, and azurite.
Editors	Elizabeth Sklute (MHC '06)

	Photomicrograph of calcite under plane polarized light using 10X magnification. Calcite is almost indistinguishable from other minerals under plane polarized light.
	Photomicrograph of calcite under cross polarized light using 10X magnification. Calcite is distinguished by its high order, often disallowed color and rhombahedral twinning.
	Photomicrograph of calcite under cross polarized light using 10X magnification. Note how the twins stand out agains the extinct crystal and the crystal is specled with 'birefringent dust'.